Bulk demagnetizer system and method

ABSTRACT

A PORTABLE BULK DEMAGNETIZER HAVING A SHAFT RECEIVING A REEL OF MAGNETIC TAPE WHICH IS ROTATED TO MOVE SUCCESSIVE RADIAL SEGMENTS OF THE TAPE THROUGH AN ALTERNATING POLARITY MAGNETIC FIELD, THE SHAFT BEING PROGRESSIVELY MOVED AXIALLY TO MOVE THE RECORD MEDIUM AWAY FROM THE MAGNETIC FIELD SOURCE WITH SUFFICIENT UNIFORMITY TO EFFECTIVELY DEMAGNETIZE THE TAPE.

United States Patent inventor Marvin Camras Giencoe, ill.

Appl. No 750,874

Filed Aug. 7, 1968 Patented June 28, 1971 Assignee ilT Research institute Chicago, 111.

BULK DEMAGNETIZER SYSTEM AND METHOD 24 Claims, 6 Drawing Figs.

U.S. Ci 317/157.5, 317/262, 179/100.2

int.Cl 1101113/00 Field of Search 317/1575,

[ 5 6] References Cited UNITED STATES PATENTS 250,273 11/1881 Maxim 1. 317/1575 507,912 10/1893 Boiteaux 317/1575 2,481,392 9/1949 Camras 3 l 7/157.5l

Primary Examiner-Robert K. Schaefer Assistant Examiner-William J. Smith Attorneyi-iill, Sherman, Meroni, Gross and Simpson ABSTRACT: A portable bulk demagnetizer having a shaft receiving a reel of magnetic tape which is rotated to move successive radial segments of the tape through an aitemating polarity magnetic field, the shaft being progressively moved axially to move the record medium away from the magnetic field source with sufficient uniformity to effectively demagnetize the tape.

BULK DEMAGNETIZER SYSTEM AND METHOD CROSS-REFERENCE TO RELATED APPLICATION In compliance with the requirement of 35 U.S.C. I20 reference is made to my copending application Ser. No. 456,192 filedMay I7, 1965, and the disclosure thereof is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION To bulk erase a reel of tape, heavy and expensive automatic equipment has been necessary. While inexpensive equipment will obliterate a previous recording, it has been found that long wavelength magnetization remains on the tape.

SUMMARY OF THE INVENTION The present invention teaches that a low cost portable bulk demagnetizer may be constructed which eliminates such long wavelength magnetization providing the reel of tape is moved sufliciently gradually and uniformly through a critical range of magnetic field intensities.

It is therefore an object of the present invention to provide a low cost bulk demagnetizer and method which yet effectively eliminates suchlong wave length magnetization.

Another object of the invention is to provide a particularly low cost readily portable bulk demagnetizer which is extremely compact.

A further object of the invention is to provide a low cost bulk demagnetizer suitable for quickly and effectively demagnetizing tapes having television recordings thereon.

A still further object of the invention is to provide a portable low cost demagnetizer which duplicates the performance of heavy and expensive automatic equipment.

Other objects, features and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is afragmentary somewhat diagrammatic vertical sectional view illustrating one form of demagnetizer apparatus in accordance with the present invention,

FIG. 2 is a somewhat diagrammatic plan view of a second form of demagnetizer in accordance with the present inven tron;

FIG. 3 is a vertical sectional view taken generally along the line III-III of FIG. 2.

FIG. 4 illustrates an optional power drive unit for the apparatus of FIGS. 2 and 3;

FIG. 5 illustrates a further embodiment of the present invention; and

FIG. 6 is a diagram illustrating the variation of magnetic field intensity as a function of distance for the embodiments of the present invention and illustrating by way of example the critical magnetic field region referred to in explaining the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I illustrates a first embodiment wherein a reel 10 has a magnetic tape record medium 11 wound thereon which is to be demagnetized. The reel 10 is detachably secured to a hub member 12 having a spring urged detent 13 for securing the reel against a shoulder portion 14.

The hub member 12 is shown as having a threaded shaft 16 depending therefrom, and which shaft extends through a threaded aperture 17 in a bracket 18. The hub member 12 is provided with a handle 20 which as hereafter explained is utilized to rotate the reel 10 and threaded shaft 16.

A mounting means is generally indicated at 22 and comprises a nonmagnetic casing part 24 enclosing an electromagnet 26 including a magnetic core 27 and a winding 28. A polar face 30 of the core means 27 may have a generally rectangular configuration and may be in vertical alignment with a generally radial segment of the coil of magnetic tape 11 carried by the reel 10. The mounting means further comprises a bushing 32 for receiving the threaded shaft I6 with sufficient clearance to accommodate rotation of the shaft 16 on its axis of rotation as well as translation of the shaft in the direction of its axis which is vertical as viewed in FIG. 1.

When the threaded shaft 16 is inserted into the bore 33 of bushing 32, bracket 18 engages sidewall 34 of the mounting means so as to prevent rotation of the bracket l8. Accordingly, when the handle 20 is manually operated, rotation of the shaft 16 will cause the reel 11 to move progressively away from the electromagnet 26 at a controlled rate which is coordinated with the rate of rotation of the reel.

It is found possible to duplicate the performance of heavy expensive automatic bulk demagnetizing equipment with the apparatus of FIG. 1. A typical unit of automatic equipment weights 90 pounds, draws 23 amps at 117 volts alternating current and erases tape to -70 to 90 decibels below saturated signal level. To achieve comparable results with the inexpensive portable unit of FIG. 1, it is found that the tape must b e moved very slowly through a range of critical filed intensities such as illustrated in FIG. 6 for the case of a magnetic tape having a coercive force of the order of 225 oersteds. For example, the handle 20 may be turned at about one-tenth tum per second in the direction which raises the reel, the slow turning being most necessary while the spool is in the critical filed region. The handle 20 can be turned faster afterward, and when the spool is raised about 2 to 3 inches above the surface of panel 24, the entire assembly 35 consisting of parts, I2, 16 and 18 which is coupled to the reel can be grasped and slowly lifted and moved until it is several feet from the electromagnet 26, after which the electromagnet is deenergized.

In operation of the embodiment of FIG. 1, the threaded shaft 16 is turned relative to bracket 18 until the hub 12 is at the lower initial position, the threaded shaft 16 being inserted into the bushing 32. The reel 10 is then mounted on the hub 12, and a suitable switch operated to energize the electromagnet with alternating current. The handle 20 is now turned slowly as the magnetic tape moves through the critical field region illustrated in FIG. 6. When the spool is raised about 2 to 3 inches above the surface of panel 24, it is grasped and the entire assembly 35 coupled therewith is slowly lifted and moved until several feet from the electromagnet, after which the electromagnet is deenergized.

In FIGS. 2 and 3, a reel 40 has magnetic tape 41 wound thereon which is to be demagnetized. The reel includes a hub portion 42 having a central aperture 43 and a pair of side flanges 44 and 45. A threaded shaft 48 has a disc 50 for supporting the reel 40 and has a threaded upper end 52 for receiving a suitable reel retainer member 54. The retainer 54 may be provided with a finger aperture such as indicated at 55 to facilitate manual rotation of the assembly including reel 40, shaft 48 and retainer 54.

A casing mounts an electromagnet 62 in alignment with a radial segment of the coil of magnetic tape 41 as in the previous embodiment. The electromagnet includes a generally U shaped magnetic core 64 having pole faces 65 and 66 defining a polar region (including the gap between the pole faces 65 and 66) which has a dimension radially of the shaft 48 substantially less than the diameter of the reel 40. A winding 70 is shown encircling one leg of the core. By way of example, the winding may be energized from a 117 volt, 60 hertz source under the control of a pushbutton switch indicated at 72.

The casing 60 may including a panel member 74 of bakelite or the like which serves to separate the undersurface of the reel 40 from the pole faces 65 and 66.

A bushing 76 with a smooth bore 77 receives the shaft 48 with sufficient clearance to accommodate rotation and translation. A release lever is indicated at 80 which is pivotally mounted at 81 to the casing 60 and is provided with a tension spring 84 for urging the lever in the clockwise direction relative to pivot 81 as viewed in FIG. 2. This lever is thus normally urged into engagement with shaft 48 to serve as a nut for cooperation with the external thread of shaft 48 while accommodating resetting of the shaft 48 to the lower initial position without requiring rotation thereof during insertion of the shaft into bushing 76.

The embodiment of FIGS. 2 and 3 may be manually powered by inserting the finger into aperture 55 and rotating the reel 40 and the associated parts 48 and 54 in the direction to gradually raise the reel 40 away from the electromagnet 62. The shaft 48 maybe provided with a smooth lower section 86 to accommodate removal of the reel and the associated parts and displacement to a distance of several feet while the electromagnet remains energized for example by maintaining pushbutton 72a of switch 72 depressed.

As an alternative, the auxiliary power drive unit 88 of FIG. 4 may be utilized to drive the shaft 48 of the embodiment of FIGS. 2 and 3. The unit 88 includes a housing 90 having an electric motor 91 which is coupled by means of suitable gearing so as to drive the hollow shaft 92. The shaft 92 is provided with an internal keyway 94. A coupling shaft 98 is reciprocally mounted in the bore of shaft 92 and is provided with a key 100 for sliding movement in the keyway 94. A compression spring 102 urges the shaft 98 upwardly so that the upper end 98a of the shaft is urged into coupling engagement with the lower end 48a of threaded shaft 48, FIG. 3, when the demagnetizer and drive unit are assembled. Specifically, it will be noted that the housing 90 is provided with a receiving well 104 which is dimensioned to receive the lower portion of the casing 60. The parts 48a and 98a may be provided with any suitable detachable coupling arrangement to enable motor 91 to drive the threaded shaft 48 when the components are in assembled relation. The casing 60 may include a suitable socket 108 and the well 104 may include a plug 110 to provide a convenient means for energizing motor 91 under the control of the pushbutton switch 72.

In operation of the power driven demagnetizer, the reel 40 is secured on the shaft 48 by means of retainer 54 with the shaft 48 at its lowermost initial position. The pushbutton 72a is then depressed to energize electromagnet 62 and motor 91. the reel being rotated at a relatively slow rate as in the previous embodiment and slowly raised in a vertical direction. When the threaded portion of shaft 48 has been disengaged from the associated portion of lever 80, the reel assembly may be grasped and removed by several feet from the electromagnet 62 before the pushbutton 72a is released to deenergize the electromagnet and the motor.

By way of example, motor 91 may drive a worm shaft 112, and the hollow shaft 92 may be provided with a worm wheel -l14 driven by the shaft 112. The coupling parts 48a and 98a may utilize a Phillips head (crossed sIot)-type coupling or other suitable disengageable coupling.

FIG. illustrates an embodiment generally similar to that of FIGS. 2 and 3 wherein a reel 140 is secured to a shaft 148 by means of a retainer 154 having a finger aperture 155. A casing 160 contains an electromagnet 162 including a magnetic core 164 and a winding 170. A nonmagnetic panel 174 of casing 160 may overlie the polar faces of magnetic core 164. Mounting means for the. reel assembly comprises a bushing 176 which accommodates rotation and vertical reciprocation of the shaft 148.

In this embodiment, a cam 180 is mounted for manual rotation on a shaft 182 and is provided with a cam face 184 of progressively increasing radius of curvature. The contour of the cam face 184 is such as to facilitate the gradual raising of .the shaft 148 during rotation of the retainer 154. By way of example, a hand operated crank (not shown) may be utilized to rotate cam 180 at a controlled rate coordinated with the rate of manual rotation of the reel 140 such that the magnetic tape of the reel 140 is completely demagnetized as explained in connection with the preceding embodiments.

The operation and desirable parameters for the embodiments of FIGS. 25 are the same as those for FIG. 1 except as specifically indicated herein.

An embodiment such as illustrated in FIG. 1, for example was tested with a seven inch diameter reel of magnetic tape having a television signal recorded thereon. The demagnetizer gave clearly erased tape with no trace of noise or nonuniformity. The erased tape was recorded and the recording played back, after which the tape was erased again. The result was a perfectly clean and uniform tape with a degree of erasure comparable to that obtained with the typical automatic equipment previously referred to herein.

In each of the illustrated embodiments a mounting means is provided (22, FIG. 1, 60, FIG. 3 and 160, FIG. 5) which provides for rotation of the reel about an axis of rotation which is disposed generally parallel to a predetermined direction of decreasing field intensity and providing for relative movement of the reel in a direction of translation generally parallel to the direction of decreasing field intensity. An operating means (parts 16, 17, FIG. 1, parts 48, 80, FIG. 3 and parts 148, 184, FIG. 5) mechanically move the reel relatively in the direction of translation during rotation of the reel about the axis of rotation of the reel about the axis of rotation to substantially completely demagnetize the magnetic record medium. The operating means in each embodiment provides for the subjection of the record medium to a magnetic field intensity which gradually diminishes from a value substantially in excess of the coercivity of the record medium to a value somewhat below the coercivity. The term coercivity as used herein refers to the coercive force as measured at 60 hertz and with an applied field amplitude corresponding to saturation flux density in the record medium. The term coercive force" as used herein may be taken as a reasonable approximation of coercivity and refers to a measurement at 60 hertz with an applied magnetizing field amplitude of 1,000oersteds. Where the coercivity of the record medium is of the order of 225 oersteds, the critical field region would extend from about 300 oersteds to somewhat below 225 oersteds. In the illustrated embodiments, with a 60 hertz alternating polarity magnetic field, the record medium is moved gradually through the critical magnetic field zone from about 300 oersteds to below 225 oersteds over a time interval of at least several seconds. After movement of the record medium through the critical field zone the reel may be removed from the vicinity of the electromagnet in the presence of the alternating polarity magnetic field at a rate of movement which subjects the record medium to a substantially more rapidly diminishing magnetic field intensity.

In each of the embodiments, a mechanical coupling is provided between the operating means and the mounting means to provide mechanically controlled gradual movement of the reel relative to the mounting means in the direction of translation through the critical portion of the magnetic field region such as indicated in FIG. 6. In FIG. 1, the operating means I6, 17 is coupled with the mounting means at 34 by means of the bracket 18, so that the coupling may be disengaged to accommodate more rapid movement of the reel in the direction of translation. In the embodiments of FIGS. 3 and 5, the portions 48, and 148, 184 of the operating means provide the mechanical coupling with the mounting means and are disengageable to accommodate more rapid movement of the reel.

By way of example of a specific embodiment, the threaded elements 16 and 48 may have a pitch of 32 threads per inch or 16 threads per inch. A rate of rotation of one revolution every I0 seconds or every 5 seconds may be used. The overall dimensions of the demagnetizer case may be 4% inches wide, 7% inches long (the horizontal dimension as viewed in FIG. 3, for example) and 3 /2 inches high, so that the maximum dimension of the casing is less than about 8 inches. For a seven inch reel fully loaded with magnetic tape, the rate of rotation should not exceed l revolution per second during exposure to the critical magnetic field zone. In the illustrated embodiments which provide repeated exposure of a given segment of the tape within the critical field zone, each exposure should involve at least 3 cycles of the alternating polarity magnetic field and preferably at least 4 cycles. In the absence of such repeated exposure within the critical field zone, exposure to at least 5 cycles of the alternating polarity magnetic field within the critical zone is considered preferable.

In each of the embodiments, an alternating polarity magnetic field is established in a magnetic field region which has a decreasing field intensity along a path which is generally vertical in the critical field region but which may typically curve as indicated at 200 in H6. 1 and still constitute a path with decreasing field intensity along which the assembly 35 may be moved after decoupling thereof from the mounting means. In each of the illustrated embodiments, the reel is rotated during movement in the direction of translation a distance of at least one inch and thereafier the reel is moved a substantially greater distance along the path of decreasing field intensity without rotation thereof in the presence of the alternating polarity magnetic field. In each of the embodiments, the movement of the reel in the direction of translation is coordinated with the rotation of the reel to subject the record medium to a magnetic field intensity which gradually diminishes from a value substantially in excess of the coercivity of the record medium to a value somewhat below the coercivity of the record medium, the reel thereafter being translated through the magnetic field and along the path at a substantially more rapid rate.

For example where the coercivity of the record medium is taken as approximately 225 oersteds, the reel is rotated at a relatively uniform rate during subjection of the record medium to magnetic field intensities which gradually diminish from about 300 oersteds to below 225 oersteds and thereafter the reel is relatively rapidly moved along the path in the presence of the alternating polarity magnetic field.

Each of the embodiments illustrates a mechanical motion reduction means for mounting the elongated magnetic record medium in a compact bulk configuration, the mechanical motion reduction means including a mechanical element such as screw thread 16 or 48, or cam surface 184 for progressively moving the magnetic record medium in the bulk configuration thereof in a translation direction having a substantial component which is parallel to the direction of decreasing field intensity, for example the vertical motion of the reel being in step with but at a greatly reduced rate in relation to the motion of the operating means such as handle 20, finger depression 55 or the operating handle for the cam 180.

It will be apparent that many modifications and variations may be effected without departing from the scope to the novel concepts of the present invention.

I claim:

1. A bulk demagnetizer comprising:

a fixed support having means for establishing in a magnetic field region an alternating polarity magnetic field having a decreasingfield intensity along a path in the field region;

said support having means for mounting a tape record medium in a compact annular record medium configuration with a central axis thereof substantially parallel to the path of decreasing field intensity and with the plane of the annular record medium configuration disposed substantially normal to said path; and

operating means for coupling with the mounting means to control the exposure of said annular record medium configuration to said alternating polarity magnetic field in said magnetic field region to effect demagnetization of said recordmedium, said operating means comprising mechanical means mechanically coupled with said mounting means and operating at a mechanically con trolled gradual rate for progressively moving the annular record medium configuration in a direction of translation generally along said path of decreasing field intensity while maintaining the plane of said annular record medium configuration essentially at right angles to said path throughout the demagnetizing operation.

2. A bulk demagnetizer comprising:

a support having a shaft operably associated therewith, and having an electromagnet disposed at one side of said shaft for producing a magnetic erasing field at one sector of the region surrounding said shaft;

said shaft having record medium mounting means for mounting a magnetic record medium in a compact annular configuration, the mounting means mounting the record medium configuration with its central axis coinciding with the axis of said shaft and with the plane of the record medium configuration disposed substantially at right angles to the axis of the shaft, and with a sector only of the record medium configuration being in axial alignment with the electromagnet with respect to a direction parallel to the axis of the shaft; and operating means providing for rotation of said shaft together with said record medium configuration while maintaining the plane of said record medium configuration at right'angles to the axis of said shaft so that successive sectors of the record medium configuration move into axial alignment with said electromagnet for exposure to'said magnetic erasing field, and said operating means comprising mechanical means operating at a mechanically controlled gradual rate for progressively reducing the intensity of the magnetic field acting on the successive sectors of said record medium configuration. 3. The demagnetizer of claim 2 with said support having a sleevelike journal constraining said shaft for rotation on its axis and for translation exclusively parallel to said axis, and said mechanical means acting on said shaft to move the shaft at said mechanically controlled gradual rate in the axial direction while constrained by said sleevelike journal to move the record medium configuration progressively away from the electromagnet at said gradual rate as the shaft rotates on its axis in said journal while the plane of the record medium configuration is continuously maintained at right angles to the shaft and with successive sectors of the record medium configuration continuing to move into axial alignment with the electromagnet as the record medium configuration is gradually moved away from the electromagnet.

4. A' demagnetizer according to claim 3 with said mechanical means providing for movement of said shaft in the axial direction a distance of the order of one inch so as to move the record medium through a critical field zone sufficiently gradually to insure substantially complete demagnetization of the record medium, said shaft together with the record medium being disengageable from said support by axial movement of the shaft out of said sleevelike journal after movement of the record medium through the critical field zone, whereby the record medium configuration may be manually grasped after movement thereof of through the critical field zone to accommodate manual translation of the shaft and record medium configuration at a substantially more rapid rate in a direction generally away from said electromagnet.

5. The method of demagnetizing a magnetic record medium which comprises:

establishing in a magnetic field region an alternating polarity magnetic field having a decreasing field intensity along a path in the field region;

mounting a reel having the magnetic record medium wound thereon for rotation on an axis of rotation which axis is generally parallel to said path with a radial segment of the magnetic record medium in said magnetic field region; and

rotating the reel on said axis of rotation to move successive radial segments of the record medium into said magnetic field region, mechanically moving the reel progressively in a direction of translation generally along said path while maintaining the plane of the reel essentially at right angles to said path, and coordinating the moving of the reel in said direction of translation with the rotation of the reel to substantially completely demagnetize the mag netic record medium.

6. The method of claim 5 further comprising rotating the reel during movement of the reel in said direction of translation a distance of at least one inch and thereafter moving the reel a substantially greater distance along said path without rotation thereof in the presence of said alternating polarity magnetic field.

7. The method of claim further comprising coordinating the moving of-the reel in the direction of translation with the rotation of the reel to subject the record medium to a magnetic field intensity which gradually diminishes from a value substantially in excess of the coercivity of the record medium to a value somewhat below the coercivity of the record medium, and thereafter translating the reel through the magnetic field and along the path at a substantially more rapid rate.

8. The method of claim 5 further comprising rotating the reel at a relatively uniform rate during subjection of the record medium to magnetic field intensities which gradually diminish from about 300 oersteds to below 225 oersteds and thereafter relatively rapidly moving the reel along said path in the presence of said alternating polarity magnetic field.

9. A bulk demagnetizer comprising:

an electromagnet including magnetic core means defining a magnetic field region and winding means coupled to said core means for producing an alternating polarity magnetic field in said magnetic field region having a decreasing field intensity in a predetermined direction;

mounting means for mounting a reel having a magnetic record medium wound thereon which is to be demagnetized and providing for rotation of the reel about an axis of rotation, said axis of rotation being disposed generally parallel to the predetermined direction of decreasing field intensity, and said mounting means providing for relative movement of the reel in a direction of translation generally parallel to said predetermined direction while maintaining the plane of reel substantially at right angles to said predetermined direction;

operating means for mechanically moving the reel relatively in said direction of translation during rotation of said reel about said axis of rotation to substantially completely demagnetize the magnetic record medium wound on said reel; and

said electromagnet being disposed at one side of said axis of rotation and being in alignment with a sector only of a reel mounted on said mounting means, rotation of the reel moving successive sectors of the reel into alignment with the electromagnet throughout the time during which the operating means mechanically moves the reel in said direction of translation, so that the successive sectors of i the reel are successively exposed to the alternating polarity magnetic field and so that a given sector of the reel moves into alignment with the electromagnet at successively increased distances in said predetermined direction from the electromagnet. 10. A demagnetizer according to claim 1 with said operating means providing for the subjection of the record medium to a magnetic field intensity which gradually diminishes from a value substantially in excess of the coercivity of the record medium to a value below said coercivity.

11. A demagnetizer according to claim 10 with said operating means serving to gradually diminish the magnetic field intensity acting on the record medium from about 300 oersteds to below 225 oersteds over a time interval of at least several seconds.

12. A demagnetizer according to claim 1 with said operating means providing for the subjection of the record medium to a magnetic field intensity which gradually diminishes from a value substantially in excess of the coercivity of the record medium to a value below said coercivity, and thereafter accommodating removal of said reel from the vicinity of the electromagnet while the electromagnet continues to produce said alternating polarity magnetic field, at a rate of movement which subjects the record medium to a substantially more rapidly diminishing magnetic field intensity.

13. A demagnetizer according to claim 1 with said operating means having a mechanical coupling with said mounting means to provide mechanically controlled gradual movement of the reel relative to said mounting means in said direction of translation through a critical portion of said magnetic field region and said mechanical coupling thereafler being disengageable to accommodate more rapid movement of said reel in said direction of translation.

14. A demagnetizer according to claim 1 with said magnetic core means having a polar region disposed generally radially relative to the axis of rotation of the reel and offset to one side thereof for subjecting successive radial segments of the record medium wound on the reel to said magnetic field as the reel is rotated.

15. A demagnetizer according to claim 1 with said operating means comprising a screw thread for coupling to the reel for rotation therewith and having a pitch to gradually move the reel in said direction of translation as the reel is rotated.

16. A demagnetizer according to claim 15 with said operating means further comprising manually operated means for coupling to said reel to rotate said reel and thereby to rotate said screw thread to progressively move the reel in said direction of translation.

17. A demagnetizer according to claim 15 with said screw thread being engaged with said mounting means to produce gradual movement of the reel through a critical portion of said magnetic field region and thereafter being disengageable from said mounting means to accommodate more rapid movement of the reel in said direction of translation.

18. A demagnetizer according to claim 1 with said mounting means having a power drive for coupling with said reel to rotate the reel at a substantially uniform rate, and said operating means having a mechanical element driven in coordinated relationship with the rotation of said reel to progressively move the reel in said direction of translation.

19. A demagnetizer according to claim 18 with said power drive being detachable from said reel after movement of the reel a first distance in said direction of displacement thereby to accommodate further manual removal of the reel away from said electromagnet in the presence of said alternating polarity magnetic field.

20. A demagnetizer according to claim 1 with an auxiliary power unit for detachable coupling with said operating means to provide for power driving of the said operating means when the power unit is .coupled with the operating means, the operating means accommodating manual operation when the power unit is detached therefrom.

. 21. A demagnetizer according to claim 1 with said operating means comprising a threaded shaftfor coupling to said reel during demagnetizing operation, said mounting mean having a power drive for coupling with said threaded shaft to rotate the same in a direction so that the shaft and reel progressively move in said direction of displacement in response to rotation thereof, and means providing for. disengagement of the threaded shaft from the power drive after the reel has moved a predetermined distance in said direction of displacement thereby to accommodate further manual removal of the reel away from the electromagnet in the presence of said alternating polarity magnetic field.

22. A demagnetizer according to claim 1 with the demagnetizer being portable, said mounting means including a casing having a panel with a shaft projecting beyond the plane of the panel for receiving the reel, said casing having said electromagnet mounted therein offset to one side of the axis or rotation of said shaft to produce the magnetic field region at the external side of said panel and intersecting a radial segment of the magnetic record medium wound on the reel, the shaft being progressively projected relative to the plane of said panel in response to rotation thereof to progressively move the reel away from the electromagnet.

23. A demagnetizer according to claim 22 with the casing having a maximum dimension which is less than about 8 inches.

24. A demagnetizer according to claim 1 with said operating means comprising a cam coupled with said reel and progressively movable to progressively move said reel in said direction of translation during rotation of the reel. 

